"The First International Symposium on Osteopetrosis: Biology and Therapy" will take place at the Natcher Center, National Institutes of Health, on October 23-24, 2003. This meeting, which is jointly sponsored by the Paget Foundation, was organized to facilitate discussion of clinical and basic research applicable to the disorders described as osteopetrosis. The intention is to bring together leaders in the field of bone biology, genetics and clinical care of these patients, recognizing that there has been significant recent gains in basic and clinical research relevant to this disorder. Objectives of this Symposium are outlined in Specific Aims. Approximately 45 persons will attend. Osteopetrosis is a heterogeneous group of inherited conditions characterized by a defect in bone resorption by osteoclasts, leading to an increase in skeletal mass and bone density. In the more severely affected individuals, the lack of osteoclast function leads to a bone marrow cavity insufficient to support hematopoiesis. Autosomal recessive osteopetrosis is fatal in the majority of patients before they reach 10 years of age, generally from marrow failure and bleeding or infections. The molecular basis of human osteopetrosis has remained elusive until very recently. It is now appreciated that several gene defects leading to an inability of osteoclasts to acidify the extracellular space are responsible for approximately 70% of occurrences of autosomal recessive osteopetrosis. However, there is little information yet available regarding the correlation of genotype and phenotype in affected individuals. In addition, it is clear that there are genetic defects that have not yet been defined. Therapeutic approaches for osteopetrosis have been limited. Interferon-7 has been shown to increase the function of osteoclasts from patients with osteopetrosis, but this is not universal, nor is it necessarily maintained. Hematopoietic cell transplantation has been shown to be effective, but it is difficult, expensive and life-threatening complications are commonly observed. Based on the new information available regarding these disorders, the goal of "The First International Symposium on Osteopetrosis: Biology and Therapy," jointly sponsored by The Paget Foundation, is to facilitate discussion of clinical and basic research applicable to the disorders described as osteopetrosis by leaders in the field of bone biology, genetics, and clinical care.